A complementary metal-oxide semiconductor (CMOS) imager circuit includes a focal plane array of pixel cells, each of the cells including a photo sensor such as, for example, a photo gate, photoconductor or a photodiode for accumulating photo-generated charge in the specified portion of the substrate. Each pixel cell has a charge storage region, formed on or in the substrate, which is connected to the gate of an output transistor that is part of a readout circuit. The charge storage region may be constructed as a floating diffusion region.
In a CMOS imager, the active elements of each pixel cell may perform the functions of: photon to charge conversion, accumulation of image charge, resetting the storage region to a known state, transfer of charge from the photo sensor to the storage region, selection of a pixel for readout, and output and amplification of a signal representing pixel charge. Photo charge may be amplified when it moves from the initial charge accumulation region to the storage region. The charge at the storage region is typically converted to a pixel output voltage by a source follower output transistor.
CMOS imagers of the type discussed above are generally known and described in, for example, U.S. Pat. No. 6,140,630, U.S. Pat. No. 6,376,868, U.S. Pat. No. 6,310,366, U.S. Pat. No. 6,326,652, U.S. Pat. No. 6,204,524 and U.S. Pat. No. 6,333,205, assigned to Micron Technology, Inc.